1. Field of the Invention
This invention relates to extension ladders. More particularly, it concerns extension ladders with automatic hook locks of the rotary type for adjustment of the length of the ladders.
2. Description of the Prior Art
It is conventional practice in the manufacture of extension ladders to use automatic hook locks to enable the position of the fly section of the ladder to be adjusted relative to the bed section. Such hook locks are most frequently of the (a) tension type or the (b) gravity type.
The tension type hook locks can be made quite compact and durable, but they are conventionally installed on the fly section with the omission of a rung in the bed section to provide space for the hook locks to operate. The omission of the rung can be avoided (see U.S. Pat. No. 968,040 and 1,457,100), but this involves specialized hooks that limit movement of the hook locks.
The gravity type hook locks are conventionally used without having a skipped rung in the fly section. Hence, they avoid the safety hazard created by the absence of the rung, but the locks of this type are excessively large and they present durability and maintenance problems.
A third variety of hook locks are the rotary type (see U.S. Pat. No. 3,507,175) which do not require the ladders to have a skipped rung. Also, they can be made more compact and are more reliable in operation than the gravity type locks. This invention concerns new improvements in the construction of this type of hook locks for extension ladders.